Strangles

Aetiology

Strangles is caused by infection with the bacteria
Streptococcus equi subspecies equi which is often
abbreviated to S.equi or Strep equi. This is a
highly contagious bacteria that survives in carrier horses and then
infects horses which come into contact with the carrier. Infection
can spread rapidly from horse to horse in populations that do not
have immunity to the bacteria. Infection occurs most commonly in
horses of 1-5 years of age but can occur in older animals. Most
horses that are exposed to infection develop a strong immunity and
are unlikely to become re-infected. Virtually all horses that are
exposed to infection become sick and some may die or be
euthanased.

Clinical signs

Following exposure to the strangles bacteria the incubation
period (the time until clinical signs develop) is 2 to 6 days. A
very high temperature (>103°F) is characteristic of infection.
Typically, Infected horses are dull, disinterested in food (and
possibly water) and develop a clear and then purulent bilateral
nasal discharge. The eyes may also look inflamed and develop a
discharge.

Within hours of infection the bacteria crosses the lining of the
respiratory tract and migrates to the lymph nodes around the head.
The bacteria have a number of mechanisms by which it resists the
host's immune system and as more and as more white blood cells move
to the lymph nodes to fight the infection abscesses develop. Lymph
node abscesses are the hallmark of strangles though they do not
develop in all cases.

Figure 1: Enlargement of the sub-mandibular
lymph nodes as a result of abscessation following Strangles
infection.

They may burst out (usually around a week after infection) and
persistently drain pus through the skin. Abscesses may also burst
inwards into the guttural pouches where pus may accumulate and dry
out to form chondroids (balls of concreted pus) which can remain in
the pouch for months or years. Enlargement of the lymph nodes at
the back of the throat may also damage the nerve supply to the
larynx and result in laryngeal paralysis causing a noise during
breathing. Enlargement may also cause compression and obstruction
of the trachea or the oesophagus preventing breathing or
swallowing, hence the name "strangles".

Figure 2: Thick purulent discharge is a
typical sign of Strangles

Figure 3: Abscess erupting from a lymph node
into the guttural pouch of a horse with
Strangles

Horses often remain infected, and infectious, for 4 to 6 weeks
but 1 in 10 will continue to discharge infectious material for
longer and may remain carriers for years acting as a source of
further outbreaks. Horses with chondroids will remain carriers
until the chondroids are removed.

Diagnosis

The clinical signs of strangles are often sufficient for
diagnosis. Vets have a saying that "if it looks like strangles then
it's strangles" which was coined because clinical signs were often
more reliable than early lab tests. Thankfully lab tests have
improved so that strangles can now be diagnosed more reliably in
less obvious cases. The most simple means of investigating
strangles is to look for antibody levels to the bacteria in the
horse's blood. If a horse has had strangles between 2 weeks and 6
months prior to testing, or is a carrier, then it will have
antibodies to S. equi and test positive on a blood test.
The blood test is accurate in 80% of cases but is not useful in an
acute case as it takes up to 2 weeks for the antibody responses to
develop.

In acute cases diagnosis can be made by identifying the bacteria
before the antibody response develops. This can be done in one of
three ways:

1. The bacteria are cultured from an abscess

2. The bacteria are cultured from a swab taken from the
back of the throat. Because the timing of swabbing has to coincide
with discharge into the throat and infection can be missed, 3 swabs
have to be collected at 5-7 day intervals. This method is therefore
laborious and doesn't give quick results when they are needed most
i.e. at the start of infection to know whether other horses are at
risk

3. Endoscopy of the throat and guttural pouches can be
performed to investigate whether there are abscesses at the back of
the throat and whether there is pus in the guttural pouches. If
nothing is visible then samples are taken from the guttural pouches
to check for bacteria. These samples are more reliable than swabs
from the back of the throat.

Although the bacteria live and multiply exceptionally well in
the horse it can be difficult to grow in the laboratory which
historically resulted in lots of negative results in horses that
did actually have the disease. Polymerase chain reaction (PCR) can
now be performed to detect bacterial DNA so even if the bacteria
die after samples are collected their presence can still be
detected. PCR has greatly improved strangles diagnosis.

Treatment

The disease is caused by bacteria and antibiotics would
therefore be a logical means of treatment. However, the use of
antibiotics is controversial as there are a number of potential
disadvantages associated with their use. Antibiotics may:

1. Prevent the development of abscesses so the infection
grumbles on for longer rather than reaching a peak and then
resolving

2. Increase the risk of abscesses developing in other
parts of the body though there is no proof of this theory.

3. Prevent development of lasting immunity so there is a
risk of re-infection

Antibiotics will usually be used:

1. Very early in infection before abscesses develop

2. If abscesses are so big that complications develop e.g.
breathing difficulties

3. In horses that develop pneumonia or other infections at
sites other than the respiratory tract

Most vets will try and avoid the use of antibiotics and will
treat horses supportively. If abscesses develop they may be
encouraged to burst by applying poultices or lancing them to allow
them to drain. The use of anti-inflammatories is important to
reduce pain and inflammation and to bring down body temperature.
Some horses (thankfully a minority) may need fluids if they are not
drinking, may require tracheotomy to enable them to breathe or may
need supplementary feeding by stomach tube. If there is
infection within the guttural pouches then the pus and chondroids
within them have to be removed before the infection will be
eliminated. This is a very time-consuming and therefore expensive
process. Surgical treatment can be required to clear the pouches
and may work out cheaper in the long term.

Disease Prevention

Strangles is a horrible disease that is associated with a huge
amount of pain and suffering in animals that become infected. A
small proportion may have to be euthanased. It is a source of
frustration to vets and equine welfare charities that the disease
appears to be becoming more prevalent because it ought to be a
disease that could be eliminated. The disease cannot survive long
in the environment and has to survive in infected carriers. If all
clinical cases were treated effectively and not allowed to mix with
other horses until they were known to be free of disease then the
disease would die-out. The disease persists because people cut
corners and allow infected horses to mix with susceptible
populations.

A strangles vaccination is available and whilst it does not
prevent infection it reduces the severity of clinical signs if
infection does occur and reduces the likelihood of disease spread.
Vaccination should not be used in an outbreak because it may
increase the risk of purpura haemorrhagica (skin disease)
developing.

When new horses enter a property it is prudent to either isolate
them for 2 weeks (some would say 4) and check for clinical signs of
strangles. Unfortunately carriers generally show no clinical signs
and few people are prepared to isolate their horses for the
required period of time. A more realistic alternative to isolation
is collection of a blood sample and measurement of antibody levels
to S.equi before horses enter a yard.

Disease Control

During an outbreak the following control measures should be
taken:

1. Isolate infected horses and horses that have had direct
contact with the infected case in the previous week. Seek
veterinary guidance on effective isolation procedures.

2. Minimise the movement of people and equipment between
horses until it is determined which horses are likely to be
infected

7. Rest stables or pastures that have contained infected
horses for at least 4 weeks

8. Demonstrate that infected horses are free from the
disease either i) by collecting 3 negative swabs from the throat at
5-7 day intervals or ii) by collecting negative culture and PCR
samples from the guttural pouches (preferably on 3 occasions but if
PCR is performed once may be acceptable).

Summary of key learning points

Strangles is caused by the bacteria Streptococcus equi
equi

Strangles is associated with respiratory infection and
abscessation of the lymph nodes around the head. This may cause
obstruction around the throat hence the name "Strangles".

Infection may spread to other sites in the body causing a range
of other diseases and clinical signs

Infection of the lymph nodes next to the guttural pouches may
result in accumulation of pus and formation of chondroids within
the pouches

Some horses will remain infected if not treated appropriately
and will act as a source of infection for other horses

The use of antibiotics is controversial. Treatment generally
comprises anti-inflammatories and supportive care.

Vaccination is of value but does not prevent disease

Affected horses and suspect cases should be isolated
immediately and veterinary guidance should be sought from the
outset on further isolation and disease control measures

Before new horses are moved onto a yard they should be isolated
or a blood sample should be taken to ensure they are not Strangles
carriers.

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